This invention relates to a method and apparatus for correcting an error which is a function of an independent variable such as errors in analytic instruments in general and more particularly to an improved system for obtaining error correction in simple fashion with a minimum of hardware.
Various types of analytic instruments have a base line which varies with a change in an independent variable. Furthermore, certain instruments have errors which change not only as a function of the independent variable but which also change over periods of time, with temperature, etc. This is a particular problem in instruments such as dual beam spectrophotometers. It is also a problem in scanning calorimeters. In the first case, an output signal error must be corrected synchronously with changes in wavelength. In the second case, corrections must be made to the output as the independent variable of temperature changes. In order to gain a better understanding of the present invention it will be disclosed in terms of a dual beam spectrophotometer. It will be understood by those skilled in the art that it may be as easily used with any type of instrument or device in which an error which is a function of an independent variable occurs.
In a dual beam spectrophotometer the concentrations of various constituents in sample substances are determined. To accomplish this two radiation beams from a single source are sequentially directed to a photo-electric detector. One of the beams, whose signal is designated I, passes through the sample. The other signal, designated I.sub.0 does not pass through the sample but provides a reference. The I beam which is passed through the sample experiences a decrease in intensity due to absorption by the constituent in proportion to its concentration in the sample. Ideally, except for the absorption by the sample, the I and I.sub.0 beam have equal intensity throughout their transmission paths. This is commonly expressed as a ratio I/I.sub.0 = 100%. However, the paths never include exactly the same optical elements since it is impossible to exactly match the reflectivity of the uncommon elements for all wavelengths of the source. In a spectrophotometer wavelengths are scanned in increments, the instrument stepping through the various wavelengths. There are variations in the transmission at each of these wavelengths. Without a correction, the base line, i.e., the zero line from which intensity is measured, is not flat and erronous results are obtained. This is a problem which has been recognized in the art and is referred to as a base line flattening problem. Various attempts have been made to solve this problem through the use of cams, tapped potentiometers or even through the use of the magnetic tape as a medium for storing an I.sub.0 correction factor which varies in synchronizm with changes in wavelength. The cam and tapped potentiometer methods are tedious to adjust and force the user to accept predetermined inflection points not too closely spaced with respect to wavelength function. The magnetic tape method is capable of automaticlly finding or adjusting the correction function and has no restriction on the occurrence of inflection points. However, it is an expensive method in view of the requirements for synchronizing the tape drive to the wavelength drive in the instrument.
Thus, there is a need for an improved base line flattening system for use in spectrophotometers, calorimeters and other similar instruments which is simple, inexpensive and does not require extensive adjustment.
In more general terms, there is a need for a simple system for correcting errors in any device where the error is a function of an independent variable in the device.